1. Field of the Invention
The present invention relates to a method of obtaining a white reference data which is used for correcting non-uniformity in photoelectric cell array, which is provided in an image scan reader such as process scanner.
2. Description of Background Arts
As is well known in the art, a linear photoelectric cell array is provided in an image scan reader for reading an original image for each scanning line. The linear photoelectric cell array has a number of photocells. The respective photoelectric output signals of the photocells are not uniform even if they read an original image having uniform optical density because of non-uniformity in sensitivity of the photocells and illumination of an original as well as fluctuation of dark currents in the photocells. In order to compensate the non-uniformity, the photoelectic signals are so corrected that uniform signals can be obtained when an original in a uniform density is read with the linear photoelectric cell array. The correction is known in the art as "shading correction".
In shading correction, a reference white plate having an uniform density is prepared, and white reference signals are generated by reading the white plane with the photoelectric cell array. A shading correction table is then produced with reference to respective levels of the white reference signals.
However, a white reference plate having complete uniformity in density is hardly ever obtained in practice, since various defects such as dusts and scratches often exist on white reference plates. Consequently, a manufacturer and a user of the image scan reader are obliged to conduct the shading correction with a defective white reference plate.
In one improvement in the shading correction process, the white reference plane is read with the linear photoelectric cell array through an optical system which is out of focus, thereby, the defects on the white reference plate are smoothed when they are projected onto the linear photoelectric cell array. However, this improvement has the disadvantage that the smoothing is not sufficient if the original image is projected onto the linear photoelectric cell array through a reduction optical system which has a large depth of field. When the white reference plate is displaced for from the original so that sufficient smoothing can be obtained, the condition of illumination on the white reference plate is different from that of the original, whereby the shading correction loses its accuracy.
In another improvement of the shading correction process, the white plate is read for a plurarity of scanning lines, to thereby obtain a plural sets of white reference signals. Then, the plural sets of the white reference signals are compared with each other, and one of them which has maximum levels is employed for the shading correction. Although the improvement has the advantage that it avoids the influence of dark defects on the white reference plate, electric noises or spurious peaks which may develop in the linear photoelectric cell array and a signal processor are fetched as "maximum levels", whereby meaningless peaks are undesirably used for the shading correction.